Acquisition and tracking system for optical radar

ABSTRACT

A laser-based tracking system is disclosed having the capability of automatically preforming an acquisition search to locate a target. In the acquisition mode, a laser beam and the instantaneous field of view of an image dissector photomultiplier are electronically and coaxially scanned over the same acquisition field. Upon acquisition of the target, the image dissector&#39;&#39;s instantaneous field of view is scanned across the target in a cross-scan pattern. The error signal is fed back to the center the laser beam and the cross-scan pattern on the target. If acquisition of the target is lost for a predetermined time the system returns to the acquisition mode to scan both the laser beam and instantaneous field of view of the image dissector over the acquisition field of view.

United States Patent Inventors 'l. O. Paine Administrator oi theNational Aeronautics and Space Administration with respect to aninvention 01; Charles L. Wymnn; John M. Gould, Huntsville, Ala. 35802;Robert E. Johnson, Williamville, N.Y. 14221; Paul F. Weiss, Sudbury,Mass. 01776 [211 App]. No. 830,366 [22] Filed June 4, 1969 [45] PatentedSept. 7, 1971 [54] ACQUISITION AND TRACKING SYSTEM FOR OPTICAL RADAR 8Claims, 4 Drawing Figs.

[52] 11.8. CI. 356/152, 250/203 X, l78/6,'l78/DIG. 21 [51] Int. Cl..G01b 11/26 [50] Field oiSearch 356/1, 141, 152; 343/6; 250/203, 199;178/6 [56] References Cited UNITED STATES PATENTS 3,504,182 3/1970Pizzurro et al. 356/141 5/1970 Whetter 3/1966 Birnbaum et a].

OTHER REFERENCES Geodolite" Laser Dist. Meas. lnstr., Model 3, Spectra-Physics, 3-1968 Primary Examiner-Rodney D. Bennett, Jr. AssistantExaminer- S. G. Buczinski Attorneys-L. D. Wotiord, Jr., A. H. Tischerand G. T. McCoy ABSTRACT: A laser-based tracking system is disclosedhaving the capability of automatically preforming an acquisition searchto locate a target. In the acquisition mode, a laser beam and theinstantaneous field of view of an image dissector photomultiplier areelectronically and coaxially scanned over the same acquisition field;Upon acquisition of the target, the image dissectors instantaneous fieldof view is scanned across the target in a cross-scan pattern. The errorsignal is fed back to the center the laser beam and the cross-scanpattern on the target. If acquisition of the target is lost for apredetermined time the system returns to the acquisition mode to scanboth the laser beam and instantaneous field of view of the imagedissector over the acquisition field of view.

AZlMUTH ELEVATION 3 TRACKING TRACKING 3| AND AND J g 34 35 i EACQUISITION ACQUISITION 2 LOGIC AND 0 c AND & =1 T 4 C T 29 CONTROLSYSTEM CONTROL SYSTEM D ll 13 27 1a 32 FF 33 IMAGE r DISSECTOR 39 k 24!I r FOCUS IMAGE Focus CONTROL DISSECTOR CONTROL PATENTEU SEP 7 I971SHEEI 2 UF 3 PATENTCD SEP 7 I97! FIG. 3

SHEET 3 OF 3 CHARLES L. WYMAN JOHN M. GOULD FIG.4

A TTORN E Y8 ORIGIN OF THE INVENTION The invention described herein wasmade in the performance of work under a NASA Contract and is subject tothe provisions of Section 305 of the National Aeronautics and Space Actof 1958, Public Law 85568 (72 Stat-435; 42 USC 2457).

BACKGROUND OF THE INVENTION This invention relates to an acquisition andtracking system. More particularly, this invention relates to anacquisition and tracking system for optical radar.

There are many advantages to an optical radar system utilizing narrowbeam radiation patterns. For example, energy is concentrated in aspecific direction thereby minimizing power requirements. Any lightsource capable of being concentrated into a well-collimated beam andmodulated can be used in this optical radar system. For example, anintense light source such as a continuous wave laser is especiallysuitable. However, the use of a narrow beam of radiation poses difficultproblems in acquisition and tracking between transmitter and receiver.Establishment of contact or acquisition requires a search both inazimuth and in elevation and the system must operate in theinstantaneous field of view defined by the divergence of the transmittedbeam.

SUMMARY OF THE INVENTION The following discussion will be directed to anacquisition and tracking system for an optical radar system wherein thelight beam may be modulated for range information. The system includes areflex unit which comprises transmitting optics for transmitting thebeam coaxially to a receiver telescope.

Acquisition, i.e., contact between the transmitter and the receiver isaccomplished by raster scanning the beam and the receiver instantaneousfield of views over a predetermined acquisition field. If the target'iswithin this acquisition field, the beam is reflected and is detectedwhen the target comes within the instantaneous field of view. Thetransmitter portion of the present invention includes a beam steererwhile the receiver includes an image dissector, both of which are slavedtogether so as to view the same instantaneous field of view during theacquisition cycle.

To maintain acquisition of the target, i.e., to track the target afterinitial acquisition, the image dissector is scanned in a cross-scanpattern to obtain a differential error signal indicating the offset ofthe target from the center of the cross-scan pattern. The differentialerror signal is fed back to force the cross-scan pattern to centeritself on the target. The instantaneous field of view of the transmitteris slaved to the center of the cross-scan pattern so that as the targetmoves about in the acquisition field, the light continues to illuminateit.

When the system is tracking a moving target, the reflex unit is placedon a tracking pedestal so as to allow the direction of the totalacquisition field of view to be controlled by a suitable servo loop.

Accordingly it is an object of the present invention to provide anacquisition and tracking system for use in optical radar.

DESCRIPTION OF THE DRAWINGS tracking cycle of FIG. I to generate adifferential error signal.

FIG. 4 are waveforms useful in understanding the generation of thedifferential error signal.

DESCRIPTIONOF PREFERRED EMBODIMENT Referring now to FIG. 1 there isshown a light source. such as a laser 11, for producing a beam of light13 that is projected through a modulator 15, a beam steerer 17 andthenthrough lenses l9 and 21 to mirror 23. The beam 13 is reflected bymirror 25 which directs the beam along the mechanical axis of a receivertelescope 27. The function of the laser is to illuminate or lightup" acorner cube reflector 29. The corner cube reflector or target is adaptedto be disposed on the object wished to be tracked and reflects lightparallel or in a rctrodirection to received light.

The modulator 15 may be of any type suitable for modulating the laseroutput at an identification frequency. The beam steerer 17 may be of thetype described by V. Fowler and J. Schlafcr in APplied Optics 5, 1975(i966) and is utilized to obtain scanning of the laser beam across apredetermined acquisition field in accordance with signals supplied bytracking and acquisition logic and control systems 31 and 32; system 31controlling the elevation of beam 13 and system 32 controlling theazimuth of beam 13. The lenses l9 and 21 are utilized to control thebeam width of the laser beam 13. I

The receiving telescope 27 includes a parabolic mirror 34 for directinglight received on the same axis as the transmitted beam 13, includingreflected beam 33 reflected from corner cube 29, to a mirror 35, thencethrough an interference filter 37 to reduce extraneous light and thenceto a beam splitter 39. The beam splitter directs a portion of the beam33 to the input of image dissectors 41 and 43. Image dissectors 41 and43 are photomultipliers having an imaging section and can scan a smallinstantaneous field of view in two dimensions. Focus circuits 45 and 47are respectively connected to the focus coils (not shown) of the imagedissectors to control the size of the instantaneous field of view.Signals applied to leads from the tracking and acquisition controlcircuits 31 and 32 respectively control the elevation and azimuth of theinstantaneous field of view of the image dissectors 41 and 43. FIG. 2shows a block diagram of the elevation tracking and acquisition logicand control system 31. System 32 is identical to system 31 except ashereinafter noted and only system 31 will be described in detail.

Referring now to FIG. 2, the elevation tracking and acquisition logicand control system has a counter 51 for providing the acquisition scanpattern control of the elevation dimension. The counter is driven byclock source 53 and has its output connected to a digital to analogconverter 55 which in turn provides a first input to a differentialamplifier 57. The counter 51 is started during the acquisition cycle bythe output from a search-track decision and switching logic circuit 59and is controlled during the tracking cycle by the output of thedifferential amplifier 57.

The output of the differential amplifier 57 is connected to the input ofintegrator 61. The output of integrator is connected to a beam deflectoramplifier 63 via preamplifier 65. The output of preamplifier is also fedback to form the second input to differential amplifier 57 so as toslave the output of the integrator to the output of the digital toanalog converter 55. The beam steerer 17 is connected to the output ofamplifier 63 and deflection monitor 69 is coupled to the output of thebeam steerer. The output of the deflection monitor provides the input toimage dissector deflection amplifier 71 so as to slave elevation scan ofthe instantaneous field of view of image dissector 41 to the elevationof the transmitted beam 13. Additionally the output of the deflectionmonitor 69 is connected to the target position circuit 73 which controlsa tracking pedestal (not shown) that determines the mechanical axis ofthe telescope 27 of FIG. 1. A tracking pattern generator 75 driven byclock 53 has its output connected to a second input of image dissectordeflection amplifier 71 and functions to provide the image dissector 41with a cross-scan pattern when the system is in the tracking cycle. Theoutput of the image dissector 41 is connected to an input thresholddeflector 77 via a narrow crystal filter 79. The range image dissector43 has its input connected to image dissector deflection amplifier 72which in turn is energized by the output of deflection monitor 69.

The output of the threshold detector 77 is fed to a monostablemultivibrator 83 thence to a track signal detector 85 and then to thesearch-track decision and switching circuit 59 which controls thetracking pattern generator. The output of the threshold detector is alsoapplied to an error signal generator 87 via gating circuit 89 having anup gate output 92 and a down gate output 94. The error signal generatorproduces an output or differential error signal representing the offsetof the comer cube 29 from the center of the cross-scan pattern. Thiserror signal is fed back to the integrator 61 and to target positioncircuit 73.

In operation, the beam width of the laser beam 13 (FIG. 1) and theinstantaneous field of view of the image dissectors 41 and 43 are madeequal by adjustment of lenses 19 and 21 and focus control circuits 45and 47. In the absence of a signal from the track signal detector 85(FIG. 2) indicating that the corner cube 29 is being tracked, thesearch-track decision circuit 59 initiates the acquisition cycle bystarting counter 51. Counter 51, which produces an output pulse after apredetermined number of clock pulses have been received from clock 53,has its output converted to an analog signal by digital to analogconverter 55 to provide a control signal for the elevation dimension ofthe beam 13 and the instantaneous field of view of the image dissector41. As previously stated it will be understood that a second controlcircuit 32 is provided to control the azimuth dimension of the beam 13and the instantaneous field of view of the image dissector 41.

In the now preferred embodiment, the method of scanning during theacquisition cycle is a step scan raster pattern over a predeterminedacquisition field. This may be accomplished by providing a digitalstaircase waveform, in the case of the elevation dimension, as thecontrol signal applied to the elevation beam deflector amplifier 63 andthe elevation image dissector deflections amplifier 71 and by providinga triangular wave form, in the case of the azimuth dimension, as thecontrol signal applied to the azimuth beam deflector amplifier and theazimuth image dissector deflection amplifier.

In a manner to be more further described hereinafter the integrator 61is a common component to both the acquisition and tracking cycles of thesystem and accepts signals from the differential amplifier 57 during theacquisition cycle and from the error signal generator 87 when the systemis tracking. So that the integrator 61 output follows the digital toanalog converter 55 output during the acquisition cycle, the output ofconverter 55 is compared with the integrator 61 output (via preamplifier65) in the differential amplifier 57 to slave the integrator 61 to theconverter 55. The deflection monitor 69 samples the input to the beamsteerer 17 to provide the means for providing a signal indicative of theelevation of beam 13 and also transfers the output of the integrator 61to the image dissector deflection amplifier 71 so that the laser beam 13and the field of view of the image dissector 41 are scanned in the sameway during the acquisition cycle and thus are always pointed on the sameaxis.

When the laser beam 13 (FIG. 1) and the instantaneous field of view ofthe image dissector 41 scan across the corner cube reflector 29, theimage dissector 41 will receive reflected beam 33. The image dissector41 in the well-known manner converts the received light beams into anelectrical signal. The output of the image dissector is filtered byfilter 79 (FIG. 2) so that only light modulated at a predeterminedidentification frequency will be tracked and the output of filter 79 isapplied to the input of threshold detector 77, which squares signalsabove a predetermined threshold and provides a control signal to theinput of gating circuit 89 and monostable multivibrator 83. Additionallyduring the acquisition cycle the output signal of the threshold detector77 is applied directly to the searchtrack decision circuit 59 toinitiate a first command signal causing counter 51 to stop and toprovide a second command signal to start tracking pattern generator 75.

The energization of the tracking pattern generator causes a signal to beapplied to the image dissector deflection amplifier 71 so as to causethe instantaneous field of view of the image dissector 41, but not beam13, to scan up and down one beam position in elevation. A similaroccurrence also occurs in the tracking and acquisition logic and controlsystem 32 that controls the azimuth of the instantaneous field of viewof the image dissector. That is, after detection of the reflected beam33 the azimuth tracking and control system 32 (FIG. 1) causes theinstantaneous field of view of the image dissector to scan back andforth one beam position thereby causing the total instantaneous field ofview of the image dissector 51 to be scanned in a cross-scan pattern.Such scanning causes the instantaneous field of view of the imagedissector 41 to move across the target or corner cube 29 therebychopping the output signal of the image dissector 41 and the inputthreshold detector 71 output signal. The chopped output signal of thethreshold detector 77 is gated by gating circuit 77 so as to produce anup gate output on lead 92 and a down gate output on lead 94 that arecombined in the error signal generator 87.

This will be more clearly understood by reference to FIGS. 3 and 4. FIG.3 is a representation of the cross-scan pattern of the instantaneousfield of view of image dissector 41 during the tracking cycle. The sizeof the instantaneous field of view or one-beam position is representedby square 91 and the method of scanning may be seen by following thepath of the star-shaped line 93. Two possible positions of the reflectedbeam 33 within the instantaneous field of view are shown by circles 95and 97. FIG. 4 discloses various waveforms used in generating thecross-scan pattern and in generating the differential error signal usedin centering the reflected beam in the cross-scan pattern.

Referring now to waveform A of FIG. 4 there is shown the output signalof the tracking pattern generator 75 of FIG. 2 that is applied to theimage dissector amplifier 71 to cause the instantaneous field of view ofthe image dissector 41 to move up and down. Waveform B shows the choppedoutput of the threshold detector 77 of FIG. 2 when the reflected beam 33is centered as at 95 in FIG. 3, while waveform F shows the choppedoutput of the threshold detector 77 when the reflected beam 33 isdisplaced downward as at 97 in FIG. 3.

Waveforms C and D represent the up gate and down gate output of gatingcircuit 89 of FIG. 2 that form the input to the error signal generator87 when the target is centered. The down gate output signal is invertedby the error signal generator 87 and added to the up gate output signalto form error signal (waveform E) that is applied to integrator 61. Itwill be apparent that since the average DC signal of the differentialerror signal is zero the output of the integrator 61 will not bealtered. Accordingly the elevation of beam 13 and the center of theinstantaneous field of view of the image dissector 41 will remain thesame.

Referring now to waveforms G and H of FIG. 4 there is shown the up anddown gate output signals when the target is displaced downward as at 97in FIG. 3. In this instance the up and down gate output signal are notequal and an average or net DC error signal (waveform I) is applied tointegrator 61. The output of the integrator accordingly will be alteredin a direction tending to center the target or reflected beam 33 in thecenter of cross-scan pattern of image dissector 41 and in the center ofthe laser beam 13.

The output of the error signal generator 87 is also applied to thetarget position circuit 73 that sums the error signal with the output ofdeflection monitor 69 to obtain an output signal indicative of angularseparation of the target or corner cube reflector 29 from the axis oftelescope 27. It will be understood that if the present invention isused in tracking a target which moves outside of the originalacquisition field of view, the output of the target position circuit 73may control a tracking pedestal so as to alter the acquisition field ofview.

While the system is in the track mode and the integrator 61 outputfollows the motion of the corner cube reflector 29, it is also necessaryto have the counter 51 follow these motions. This will insure that ifacquisition is lost and the system must reacquire, scanning will beginfrom the same point where it had been tracking. In the now preferredembodiment the polarity of the differential amplifier 57 is sensed bycounter control circuit 52 to produce a control signal to set thedirection of the counter 51.

To enable the system to return to the acquisition cycle, i.e., iftracking of the target is interrupted. the chopped output of thethreshold detector 77 is applied to fire monostable multivibrator 83.The track signal detector may comprise a threshold detector that detectsthe output pulses of multivibrator 83, to maintain the search trackdecision circuit 59 in the track mode and in the absence of such pulsesreturns the circuit 59 to the acquisition mode after a predeterminedtime.

While a particular embodiment of the present invention has been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from theinvention.

What is claimed is:

1. In a target acquisition and tracking system having a laser source fortransmitting a laser beam, a modulator for modulating said transmittedlaser beam at a predetermined frequency, and a beam steerer for scanningsaid modulated beam in search of a target, the improvement comprising:

a target adapted to reflect said laser beam received thereon in aretrodirection,

a telescope means having optic means associated therewith for directingsaid transmitted laser beam from said beam steerer along the mechanicalaxis of said telescope means,

said beam steerer adapted to direct said laser beam over a predeterminedacquisition field in search of said target,

said telescope means having optics for receiving said reflected laserbeam from said target,

a first image dissector associated with said telescope means,

said telescope means having additional optic means adapted to directsaid reflected laser beam toward the imaging section of said first imagedissector,

said first image dissector having a total field of view corresponding tosaid acquisition field of view, and an instantaneous field of view ofabout the size of the transmitted laser beam,

a control system having an acquisition means for generating a firstcontrol signal for causing said first image dissector to raster scan itsinstantaneous field of view over its total field of view in search ofthe received reflected laser beam, and a second control signal to causesaid beam steerer to direct said transmitted laser beam in a raster scanof said acquisition field of view in search for said target, bothsignals being slaved together so the total field of view of the firstimage dissector and said acquisition field of view are scanned coaxiallyin a corresponding manner,

said first image dissector producing a tracking signal when the receivedreflected laser beam is detected within the total field of view,

said control system having a tracking control means responsive to saidtracking signal of said image dissector for causing said first imagedissector to. stop raster scanning and to start scanning itsinstantaneous field of view in a cross-scan pattern,

said control system generating error signals representing the offset ofthe received reflected laser beam, and thus the target, from the centerof said cross-scan pattern of said first image dissector,

said control means being responsive to said error signals so as to causesaid first image dissector to center said crossscan pattern on saidreflected laser beam regardless where it is in said total field of view,and

said control means being responsive to said error signals to also causesaid beam steerer to center said transmitted laser beam on said target.l 2. A target acquisition and tracking system defined by claim 1wherein:

said control means has a track signal detector means responsive to apredetermined time duration interruption of said tracking signal forstopping the cross-scan of said instantaneous field of view of saidfirst image dissector and starting an acquisition raster scan of saidinstantaneous field of view of said first image dissector and saidtransmitted laser beam. 3. A target acquisition and tracking systemdefined by claim 1 including:

means for positioning said telescope means so as to being said targetwithin an acquisition field of view of said transmitted laser beam, andA said control means in response to said error signals causing saidmeans for positioning said telescope means to tend to center saidreflected laser beam in the total field of view of said image dissectorand thus to center the target in the center of said acquisition field ofview. 4. A target acquisition and tracking system according to claim 1including:

a filter for the electrical output of said imagedissector so that only alaser light modulated at a predetermined frequency will be tracked.

5. A target acquisition and tracking system according to claim 1wherein: 7

said cross-scan pattern of said first image dissector is up and down oneinstantaneous field of view position in elevation, and back and forthone instantaneous field of view in azimuth.

6. A target acquisition'and tracking system according to claim 1wherein:

said telescope means optics for receiving said reflected laser beam is arear parabolic mirror adapted to receive said reflected laser beam fromsaid target and direct it forward along the mechanical axis of saidtelescope toward said additional optic means.

7. A target acquisition and tracking system according to claim 1,including:

a second image dissector associated with said telescope means, and

said additional optic means of said telescope means having a beamsplitter for directing a portion of the reflected laser beam toward theimaging section of beam, second image dissector.

8. A target acquisition and tracking system according to claim 1 whereinsaid control system includes:

first control circuits for controlling the elevation of theinstantaneous field of view of said first image dissector and theelevation of said transmitted laser beam, and

second control circuits for controlling the azimuth of the instantaneousfield of view of said first image dissector and the azimuth of saidtransmitted laser beam.

1. In a target acquisition and tracking system having a laser source fortransmitting a laser beam, a modulator for modulating said transmittedlaser beam at a predetermined frequency, and a beam steerer for scanningsaid modulated beam in search of a target, the improvement comprising: atarget adapted to reflect said laser beam received thereon in aretrodirection, a telescope means having optic means associatedtherewith for directing Said transmitted laser beam from said beamsteerer along the mechanical axis of said telescope means, said beamsteerer adapted to direct said laser beam over a predeterminedacquisition field in search of said target, said telescope means havingoptics for receiving said reflected laser beam from said target, a firstimage dissector associated with said telescope means, said telescopemeans having additional optic means adapted to direct said reflectedlaser beam toward the imaging section of said first image dissector,said first image dissector having a total field of view corresponding tosaid acquisition field of view, and an instantaneous field of view ofabout the size of the transmitted laser beam, a control system having anacquisition means for generating a first control signal for causing saidfirst image dissector to raster scan its instantaneous field of viewover its total field of view in search of the received reflected laserbeam, and a second control signal to cause said beam steerer to directsaid transmitted laser beam in a raster scan of said acquisition fieldof view in search for said target, both signals being slaved together sothe total field of view of the first image dissector and saidacquisition field of view are scanned coaxially in a correspondingmanner, said first image dissector producing a tracking signal when thereceived reflected laser beam is detected within the total field ofview, said control system having a tracking control means responsive tosaid tracking signal of said image dissector for causing said firstimage dissector to stop raster scanning and to start scanning itsinstantaneous field of view in a cross-scan pattern, said control systemgenerating error signals representing the offset of the receivedreflected laser beam, and thus the target, from the center of saidcross-scan pattern of said first image dissector, said control meansbeing responsive to said error signals so as to cause said first imagedissector to center said cross-scan pattern on said reflected laser beamregardless where it is in said total field of view, and said controlmeans being responsive to said error signals to also cause said beamsteerer to center said transmitted laser beam on said target.
 2. Atarget acquisition and tracking system defined by claim 1 wherein: saidcontrol means has a track signal detector means responsive to apredetermined time duration interruption of said tracking signal forstopping the cross-scan of said instantaneous field of view of saidfirst image dissector and starting an acquisition raster scan of saidinstantaneous field of view of said first image dissector and saidtransmitted laser beam.
 3. A target acquisition and tracking systemdefined by claim 1 including: means for positioning said telescope meansso as to being said target within an acquisition field of view of saidtransmitted laser beam, and said control means in response to said errorsignals causing said means for positioning said telescope means to tendto center said reflected laser beam in the total field of view of saidimage dissector and thus to center the target in the center of saidacquisition field of view.
 4. A target acquisition and tracking systemaccording to claim 1 including: a filter for the electrical output ofsaid image dissector so that only a laser light modulated at apredetermined frequency will be tracked.
 5. A target acquisition andtracking system according to claim 1 wherein: said cross-scan pattern ofsaid first image dissector is up and down one instantaneous field ofview position in elevation, and back and forth one instantaneous fieldof view in azimuth.
 6. A target acquisition and tracking systemaccording to claim 1 wherein: said telescope means optics for receivingsaid reflected laser beam is a rear parabolic mirror adapted to receivesaid reflected laser beam from said target and direct it forward alongthe mechanical aXis of said telescope toward said additional opticmeans.
 7. A target acquisition and tracking system according to claim 1,including: a second image dissector associated with said telescopemeans, and said additional optic means of said telescope means having abeam splitter for directing a portion of the reflected laser beam towardthe imaging section of beam, second image dissector.
 8. A targetacquisition and tracking system according to claim 1 wherein saidcontrol system includes: first control circuits for controlling theelevation of the instantaneous field of view of said first imagedissector and the elevation of said transmitted laser beam, and secondcontrol circuits for controlling the azimuth of the instantaneous fieldof view of said first image dissector and the azimuth of saidtransmitted laser beam.